Plural-beam color picture tube with improved magnetic convergence structure

ABSTRACT

In a shadow mask type color picture tube comprising a maskscreen unit, electron gun means for projecting a plurality of electron beams along separate paths toward the mask-screen unit, and magnetic convergence pole pieces mounted on opposite sides of each beam for producing magnetic convergence fields extending between the pole pieces, the ends of the pole pieces nearest the beam paths are formed with flanges extending toward each other to form a restricted gap. The distance between the gap and the beam path is at least equal to one-half of the spacing between the pole pieces at the beam path and also at least equal to 1.5 times width of the gap, to minimize the production of undesirable magnetic flux extending transverse to the convergence flux in the beam path.

United States Patent Evans, Jr.

[ Oct. 28, 1975 PLURAL-BEAM COLOR PICTURE TUBE WITH IMPROVED MAGNETIC CONVERGENCE STRUCTURE Related US. Application Data Continuation of Ser. No. 89,574, Nov. 16, 1970, abandoned.

[75] Inventor:

US. Cl 313/412; 313/428 Int. Cl. H01J 29/51; I-lOlJ 31/20 Field of Search 313/69 C, 70 C, 75, 79,

313/84 (U.S. only) References Cited UNITED STATES PATENTS 3,512,035 Egawa et al. 315/13 /zr eff 119 Yoshida et al. 313/69 C Primary ExaminerRobert Segal Attorney, Agent, or Firm-Glenn I-I. Bruestle [5 7] ABSTRACT In a shadow mask type color picture tube comprising a mask-screen unit, electron gun means for projecting a plurality of electron beams along separate paths toward the mask-screen unit, and magnetic convergence pole pieces mounted on opposite sides of each beam for producing magnetic convergence fields extending between the pole pieces, the ends of the pole pieces nearest the beam paths are formed with flanges extending toward each other to form a restricted gap. The distance between the gap and the beam path is at least equal to one-half of the spacing between the pole pieces at the beam path and also at least equal to 1.5 times width of the gap, to minimize the production of undesirable magnetic flux extending transverse to the convergence flux in the beam path.

6 Claims, 9 Drawing Figures U.S. Patent Oct.28, 1975 Sheet 1 of3 3,916,244

lap/1,

BACKGROUND OF THE INVENTION Plural-beam color picture tubes generally use a plurality of pairs of internal magnetic pole pieces, one pair for each beam, coupled to external electromagnets for both static and dynamic convergence of the electron beams. US. Pat. No. 3,268,753, of Richard H. Hughes, illustrates a typical delta-type three-beam arrangement in which the pole pieces are mounted on and extend inwardly from a cylindrical non-magnetic convergence cup toward but spaced from a central Y-shaped magnetic shield, on opposite sides of the three beam paths. A magnetic convergence field extending perpendicularly between the pole pieces deflects the beam radially, inwardly or outwardly, depending on the direction of the field. The convergence pole pieces are usually spaced as far as possible from the horizontal and vertical deflection, or scanning, yoke, to minimize coupling of the'yoke fields into the pole pieces and consequent beam spot distortion. However, such spacing is undesirable, particularly for short tubes with wide-angle deflection, such as those designed for use in portable receivers.

SUMMARY OF THE INVENTION An object of the'present invention is to minimize the coupling of the magnetic deflection yoke field with the beam paths in the regions between the magnetic convergence pole pieces in, a plural-beam color picture tube.

In accordance with the invention, ina pluralbeam magnetic convergence structure comprising a pair of magnetic pole pieces straddling each beam path to produce a magnetic convergence field extending between the pole pieces, the pole pieces are provided with means for minimizing the production of undesirable magnetic flux extending transverse to the convergence flux in the beam path. For this purpose, the ends of the pole pieces nearest to the beam path may be formed with flanges extending toward each other to form a restricted gap therebetween. The distance between the gap and the beam path is equal to or greater than onehalf of the spacing between the pole pieces at the beam path, and also equal to or greater than 1.5 times the gap width.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a partial longitudinal section view with parts broken away of a three-beam color picture tube embodying the invention.

FIG. .2 is an enlarged transverse section view of the magnetic convergence assembly of FIG. 1, taken on the line 2 2 thereof.

' FIGS. 3, 4 and 5 are schematic views illustrating the operation of the convergence assembly of FIGS. 1 and 2 as compared to the operation of a conventional convergence assembly. i 4

FIG. 6 is a viewsimilarfto FIG. 5, of a modification.

FIG. 7 is an elevation viewlof the pole pieces of FIG. 6 taken in the direction 77 thereof. 1

FIGS. 8 and 9 are graphs showing the variation of magnetic convergence flux and magnetic deflection leakage flux, respectively, in a radial direction in the neighborhood of the beam path under five different pole piece conditions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1, by way of example, the invention is shown as embodied in a three-beam shadow mask type color picture tube 10. The tube 10 comprises an envelope which includes a neck 12, a funnel 13, a faceplate l3,

and a stem structure 15. A plurality of lead-in pins 16, over which an indexing wafer base 17 is disposed, are sealed through the stem 15. A mosaic dot phosphor screen 18, which may be aluminized according to known practices, is disposed on the inner surface of the faceplate 14. An apertured shadow mask 20 having an array of apertures therethrough, which is related to the array of phosphor dots of the mosaic screen 18, is mounted adjacent to the screen. i

Means in the form of a unitary electron gun assembly 22 comprising threesubstantially identical electron guns is disposed in the neck 12 and adapted to project three separate electron beams through a beam deflection zone 24 toward the mosaic screen 18. A magnetic deflection yoke 25 is provided at the junction of the neck 12 and funnel 13 for establishing horizontal and vertical beam deflecting fields in the deflection zone 24 for scanning the three beams in a raster over the mask 20 and screen 18. The three electron guns of the assembly 22 are disposed symmetrically about the central longitudinal axis of the tube 10 in an equilateral triangular (delta)array. Each of the three guns comprises a cathode assembly 26, a control grid 27, a screen grid 28, a focus electrode 29, and an anode 30, all of which are mounted in axially aligned relationship along three insulator, e.g., glass, support rods 32. The cathode assembly 26, control grid 27 and screen grid 28 are supported from the rods 32 by integral strap-like tabs 42. The focus electrodes 29 and anodes 30 are supported from the rods 32by straps 34 and 35, respectively.

A beam convergence cage 44 is mounted on the anodes 30 and cooperates with three external electromagnets 46 to establish and maintain convergence of the three electron beams at the screen 18 at all times during their scanning movements.

Heater filaments 48 for the cathodes are-supported from the rods 32 by a sectioned strap 50. The lead-in conductors 16 are connected by connectors .(not shown) to the filaments 48, cathode assemblies 26, control grids 27, screen grids 28 and focus electrodes 29 of the electron guns. An electrically conductive coating 52 on the inner wall of the funnel 13 extends from the screen 18 into' the neck '12 where it makes contact with a plurality of spring bulb spacers 54 mounted on the convergence cage 44. An ultor potential applied to the coating'52 through a lead-in terminal in the wall of the funnel l3, schematically illustrated by the arrow 56, is thus applied to the screen 18, convergence cage 44 and anodes 30.

FIG. 2 illustrates the details of the beam convergence cage 44, which embodies'the present invention, and the external electromagnets 46 which cooperate therewith to maintain proper convergence of the three beams at the screen 18. The convergence cage 44 comprises a cup of non-magnetic metal having a cylindrical wall 112 and an end wall 114. A Y-shaped magnetic shield 115 divides the cup into three parts. The end wall 114 is formed with three beam apertures 116, one in each of the three divisions of the cup 110, aligned with the apertures in the electrodes 27, 28, 29 and 30 of the three guns. The cylindrical wall 112 is formed with three pairs of longitudinal slots through which three pairs of magnetic pole pieces 118 extend. Each pole piece 118 includes a plate-like element 119 and a coupling flange 120. The elements 119 of each pair of pole pieces are substantially parallel to each other and straddle the beam path of one of the electron guns. The flanges 120 of the pole pieces 118 extend circumferentially along the external surface of the cup wall 1 12, for coupling through the glass neck 12 to the external electromagnets 46.

In accordance with the present invention, the inner ends of each pair of pole pieces 118 are provided with integralflanges 130 extending toward each other to form a restricted gap 131. The purpose of the flanges is to prevent or at least minimize the production of undesirable stray of leakage magnetic flux in the path of the beam between the pole pieces 118, as for example, that caused by magnetic flux from the scanning yoke 25 coupling to the pole pieces 118.

FIG. 3 shows the desired operation of two conventional pole pieces 118' (without flanges on the inner ends thereof) and a coupled electromagnet 46 in extablishing a convergence magnetic field having flux lines 132 extending perpendicularly between the pole pieces to'deflect (converge) the beam inwardly toward the other beams, for the particular polarity shown. The two legs of the electromagnet 46, as well as the two pole pieces 118', have opposite polarities. FIG. 4 shows schematically the effect on the structure of FIG. 3 of the stra'y or leakage flux from the adjacent scanning yoke 25 (FIG. 1), or other stray magnetic field, in the absence of the normal magnetic convergence field. As shown, the scanning flux 133 passes through the electromagnet46 and pole pieces 118, producing like polarities NN in the pole pieces 118'at the beam path. Moreover, with straight pole pieces as shown, this leakage flux fringes inwardly at 135 across the beam path (as well as outwardly) to the Y-shield l 15, in directions transverse to the convergence flux 132 of FIG. 3. The

intensity of this fringing flux is small compared to the convergence flux 132 on which it is superposed, but still is sufficient to produce appreciable distortion of the beam spot on the screen 118.

FIG. 5'shows how the inward fringing (135) of the stray or leakage flux of FIG. 4 isminimized in the present invention by providing the flanges 130 on the inner ends of the pole pieces 118 to form the restricted gap 131. Most of the flux that would otherwise fringe inwardly across the beam path is caused to follow the low reluctance path within the pole piece elements 119 and end flanges 130 to the restricted gap 131.

A modification of the pole pieces of FIG. 2 is shown in FIGS. 6 and 7. In this embodiment the pole pieces 118 are provided with integral end flanges 136 and 137 terminating in interfitting sawtooth edges 138 and 139 spaced apart to form a' restricted zig-sag gap 140, shown best in FIG. 7.

FIG. 8 is a graph showing five curves of magnetic convergence field intensity plotted against inward radial distance along a radial plane passing through the beam path, for the following conditions: (I) no pole pieces; (2) conventional straight pole pieces, as in FIG.

3; (3) flanged pole pieces with straight edges, as in FIGS. 2 and 5; (4) flanged pole pieces with interfitting sawtooth edges, as in FIGS. 6 and 7; and (5) pole pieces connected together at the inner end by a solid magnetic wall. The curves were plotted from the results of experiments conducted with enlarged mock-up structures of similar dimensions subjected to the same external convergence field, and show the field distribution along approximately the inner half of the length of the pole pieces. As to be expected, the curves for (l) and (5). are of relatively low field intensity. Curve (2) has the highest field intensity and falls off somewhat toward the open inner end of the conventional polepieces. Curves (3) and (4) have'lower field intensities at the beam path, but increase toward the restricted gap.

FIG. 9 is a graph showing five curves of magnetic leakage field plotted against inward radial distance for the same conditions as F IG. 8. Curves (3) and (4) show that the provision of the gap-restricting flanges reduces the leakage field at the beam path to only a small fraction of that measured with conventional pole pieces, curve (2). Curves (3) of FIGS. 8 and 9 were obtained with pole pieces having a straight gap with a width equal to about one-third of the distance between the pole pieces at the beam path, and also about twothirds of the distance between the gap and the beam path, in which case, the gap-beam path distance was about one-half of the pole piece spacing. If the gap width is made larger than one-third the pole piece spacing, for a given gap-beam distance, the convergence curve (3) of FIG. 8 will be raised and flattened at the beam path but the leakage curve (3) of FIG. 9 will also be raised somewhat. On the other hand, for a given gap width, the gap-beam distance can be increased or decreased considerably, as by changing the location of the beam path, without appreciably changing the leakage .flux at the beam path. I-Iowever, decreasing the gap-beam distance. reduces the uniformity of the convergence field over the beam cross-section. Therefore, for a straight gap, as shown in FIGS. 2 and 5, the gapbeam pathdistance should be equal to or greater than one-half of the pole piece spacing, and also equal to or greater than 1.5 times the gap width. As an example, a color picture tube with the new structure and magnetic convergence structure of FIGS. 1 and 2 may have the following dimensions (in inches), as compared with a conventional tube:

New Conventional L Length of pole pieces .357" .312" S Spacing between pole pieces .150" .150" D Gap beam distance .105" .058" W Gap width .050" .150" Ratio D/S .7 .387 Ratio D/W 2.l .387 Beam diameter at pole pieces .050" .050"

three electron guns in a common plate. Also, the convergence cage may be eliminated, with the pole pieces mounted directly on the ends of the final gun electrodes, as in a copending application, Ser. No. 857,371, of Richard H. Hughes, filed Sept. l2, 1969. I

I claim:

1. An assembly comprising electron gun means 'including at least two cathodes and a plurality of apertured electrodes for producing a plurality of electron beams along separate paths in the same general direction; and magnetic convergence structure adjacent to said electron gun means, said convergence structure comprising a plurality of pairs of elongated magnetic pole pieces with each pair of pole pieces extending transverse to and straddling a different one of said electron beam paths, each pair of pole pieces comprising means for coupling said pole pieces to an external magnet to produce a magnetic convergence field extending substantially perpendicularly between said pole pieces; said pole pieces further comprising plate-like elements and means for minimizing the production of undesirable magnetic flux extending transverse to said convergence field and in said beam path; said minimizing means comprising end flanges on said pole pieces remote from said coupling means and extending toward each other to form a restricted gap therebetween, the distance (D) between said gap and said beam path being at least equal to one-half of the spacing (S) between said pole pieces at the beam path and also at least equal to 1.5 times the width (W) of said gap.

2. An assembly as in claim 1, wherein said electron gun means produces three electron beams in a delta array, and said convergence structure includes three pairs of said pole pieces.

3. An assembly as in claim 2, wherein said convergence structure further includes a cylindrical cup of nonmagnetic material mounted on the final electrodes of said electron gun means and having three pairs of longitudinal slots in the cylindrical wall thereof and three beam apertures in the end wall thereof disposed substantially coaxial with said electron beam paths, said pairs of pole pieces extending through said pairs of slots and having flanges on the outer ends thereof extending circumferentially along the outer surface of said cup and constituting said means for coupling to said external magnets.

4. An assembly as in claim 1, wherein said flanges have sawtooth edges that are interfitted with each other to form a zig-zag gap therebetween.

5. A magnetic convergence cage for converging the electron beams of a plural-beam cathode ray tube,

comprising a cup-shaped member of non-magnetic material having a plurality of pairs of longitudinal slots in the side wall thereof and a like plurality of beam apertures in the end wall thereof, a like plurality of pairs of magnetic pole pieces, each comprising a plate-like element with a flange extending therefrom at one end thereof, the plate-like elements of each pair of said pole pieces extending through one pair of said slots into said cup substantially parallel to each other and straddling a different one of said beam apertures, each flange extending along the outer surface of said side wall, for coupling to an external electromagnet, the other ends of each pair of plate-like elements extending beyond the associated beam aperture and terminating in integral flanges extending toward each other to form a restricted gap, the distance between said gap and said beam aperture being at least equal to one-half of the spacing between said plate-like elements at said beam aperture and also at least equal to 1.5 times the width of said gap.

6. An assembly comprising gun means including at least two cathodes and a plurality of apertured electrodes for producing a plurality of electron beams along separate paths in the same general direction; and magnetic convergence structure adjacent to said electron gun means, said convergence structure comprising a plurality of pairs of closely spaced elongated platelike magnetic pole pieces with each pair of pole pieces extending transverse to and straddling a different one of said electron beam paths, each pair of pole pieces including means for coupling said pole pieces to an external magnet to produce a magnetic convergence field extending substantially perpendicularly between said pole pieces; said pole pieces of each pair further including end flanges remote from said coupling means, said flanges extending toward each other to form a restricted gap therebetween, said convergence structure further including a shield having platelike sections extending between adjacent pairs of pole pieces, and the distance (D) between said gap and said beam path being at least equal to one-half of the spacing (S) between said pole pieces at the beam path and also at least equal to 1.5 times the width (W) of said gap,

whereby inward fringing of leakage magnetic flux from said pole pieces across the beam path to said shield is substantially prevented by causing the leakage flux to follow alower reluctance path in said pole pieces to said restricted gap and then to said shield. 

1. An assembly comprising electron gun means including at least two cathodes and a plurality of apertured electrodes for producing a plurality of electron beams along separate paths in the same general direction; and magnetic convergence structure adjacent to said electron gun means, said convergence structure comprising a plurality of pairs of elongated magnetic pole pieces with each pair of pole pieces extending transverse to and straddling a different one of said electron beam paths, each pair of pole pieces comprising means for coupling said pole pieces to an external magnet to produce a magnetic convergence field extending substantially perpendicularly between said pole pieces; said pole pieces further comprising plate-like elements and means for minimizing the production of undesirable magnetic flux extending transverse to said convergence field and in said beam path; said minimizing means comprising end flanges on said pole pieces remote from said coupling means and extending toward each other to form a restricted gap therebEtween, the distance (D) between said gap and said beam path being at least equal to onehalf of the spacing (S) between said pole pieces at the beam path and also at least equal to 1.5 times the width (W) of said gap.
 2. An assembly as in claim 1, wherein said electron gun means produces three electron beams in a delta array, and said convergence structure includes three pairs of said pole pieces.
 3. An assembly as in claim 2, wherein said convergence structure further includes a cylindrical cup of nonmagnetic material mounted on the final electrodes of said electron gun means and having three pairs of longitudinal slots in the cylindrical wall thereof and three beam apertures in the end wall thereof disposed substantially coaxial with said electron beam paths, said pairs of pole pieces extending through said pairs of slots and having flanges on the outer ends thereof extending circumferentially along the outer surface of said cup and constituting said means for coupling to said external magnets.
 4. An assembly as in claim 1, wherein said flanges have sawtooth edges that are interfitted with each other to form a zig-zag gap therebetween.
 5. A magnetic convergence cage for converging the electron beams of a plural-beam cathode ray tube, comprising a cup-shaped member of non-magnetic material having a plurality of pairs of longitudinal slots in the side wall thereof and a like plurality of beam apertures in the end wall thereof, a like plurality of pairs of magnetic pole pieces, each comprising a plate-like element with a flange extending therefrom at one end thereof, the plate-like elements of each pair of said pole pieces extending through one pair of said slots into said cup substantially parallel to each other and straddling a different one of said beam apertures, each flange extending along the outer surface of said side wall, for coupling to an external electromagnet, the other ends of each pair of plate-like elements extending beyond the associated beam aperture and terminating in integral flanges extending toward each other to form a restricted gap, the distance between said gap and said beam aperture being at least equal to one-half of the spacing between said plate-like elements at said beam aperture and also at least equal to 1.5 times the width of said gap.
 6. An assembly comprising gun means including at least two cathodes and a plurality of apertured electrodes for producing a plurality of electron beams along separate paths in the same general direction; and magnetic convergence structure adjacent to said electron gun means, said convergence structure comprising a plurality of pairs of closely spaced elongated plate-like magnetic pole pieces with each pair of pole pieces extending transverse to and straddling a different one of said electron beam paths, each pair of pole pieces including means for coupling said pole pieces to an external magnet to produce a magnetic convergence field extending substantially perpendicularly between said pole pieces; said pole pieces of each pair further including end flanges remote from said coupling means, said flanges extending toward each other to form a restricted gap therebetween, said convergence structure further including a shield having platelike sections extending between adjacent pairs of pole pieces, and the distance (D) between said gap and said beam path being at least equal to one-half of the spacing (S) between said pole pieces at the beam path and also at least equal to 1.5 times the width (W) of said gap, whereby inward fringing of leakage magnetic flux from said pole pieces across the beam path to said shield is substantially prevented by causing the leakage flux to follow a lower reluctance path in said pole pieces to said restricted gap and then to said shield. 